It is quite useful because it gives a larger defense bonus than any of the items separately. There is a 15% Attack and Strength bonus from the Black mask that only appears during a Slayer assignment. This will give you a boost in the max hit you can do against a monster. You can acquire the knowledge on how to make this item from any of the Slayer masters.

And as per usual, the items that go into the actual helmet continue to work against monsters that require say earmuffs or a face mask to kill.